Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced distance vector interior routing protocol (IGP) which is used to find the best route to reach a subnet by performing calculations based on bandwidth and delay. It is a Cisco proprietary protocol, but in 2013 Cisco made it an open protocol so that it can be used on devices of other vendors as well, though its advanced features are still maintained and controlled by Cisco. It supports routing for both Ipv4 and Ipv6 network and also supports variable length subnet mask (VLSM). It sends routing updates to the multicast IP address and supports fast convergence. The default administrative distance of EIGRP is 5 for summary routes, 90 for internal routes, and 170 for external routes. EIGRP, when implemented, forms three tables – neighbor table, topology table, and routing table. Though it is very easy to implement, we can still encounter problems in its working.
The most common issue in the working of EIGRP is that neighbor relations are not established properly. Use the show eigrp neighbors command to see the neighbour information, which can be used to identify the problem. EIGRP neighbour relationships will not form over a secondary network, so make sure they are formed over the same primary network. Make sure the MTU size is the same for both the end devices, if they are the same, then the problem lies with the cabling or the interfaces. Use the show ip eigrp interface command to verify the configurations set on the interfaces. The interfaces on both ends should be assigned an address from the same network for them to communicate. There can also be ACLs configured on the interfaces which might be hindering the data flow to the neighbour. The hello intervals and hold timers must match for the relationship to establish. When there are other protocols as well working on the same network, then the default metric must be specified. To verify the routes are present in the routing table using the show ip route eigrp command. If the external routes are missing from the routing table make sure the originating router does not have the same router-id as the local router. If the local router has an inbound distribute list or the neighbouring router has an outbound distribute list, then it might be denying the routes. You must modify the list as necessary. If the missing routes are a part of a discontinuous network then we must disable the auto-summarization. There can be a problem of SIA (Stuck-in-Active) in which the EIGRP router does not get a reply from the neighbour in the allotted time. Then EGIRP clears the neighbours that did not reply. It can be due to many reasons, like congested links, memory shortage, flapping routes, bandwidth value misconfiguration, etc. all these must be confirmed to make sure the neighbours are not removed from the table.